Induction of type B synoviocyte-like cells from plasmacytoid dendritic cells of the bone marrow in rheumatoid arthritis and osteoarthritis

We examined the capacities of bone marrow (BM) plasmacytoid dendritic cells (pDC) to differentiate into type B synoviocyte-like cells. BM aspiration samples were obtained from 24 rheumatoid arthritis (RA) patients and 19 osteoarthritis (OA) patients during joint operations from the iliac crest. CD34+ cells and pDC purified from BM mononuclear cells were cultured with or without SCF, GM-CSF, and TNF-α for 2–4 weeks. RA BM pDC as well as OA BM pDC comparably differentiated into fibroblast-like cells (FLC), expressing cadherin-11 and producing MMP-1, especially in the presence of TNF-α. Of note, depletion of BDCA4+ pDC from RA BM CD34+ cells significantly diminished their capacities to differentiate into FLC, which were restored by addition of BDCA4 + cells in a dose–response manner. These results indicate that pDC is one of the progenitors of type B synoviocytes, suggesting that BM pDC might be involved in the pathogenesis of RA and OA.